1. Field of the Invention
The present invention relates to a device for sensing a sound pressure by means of using a sensing assembly composed of a piezoelectric substrate, an input interdigital transducer, a first- and a second output interdigital transducers.
2. Description of the Prior Art
There are two types, that is, a touch-type and an untouch-type, of conventional devices for sensing a sound pressure. For example, an electric micrometer for measuring a minute displacement, a linear scale for a large displacement, and a rotary encoder for a rotation displacement belong to the touch-type of device. The electric micrometer and the linear scale is used as, for example, a reference for measuring the length of a material. The rotary encoder is used for controlling a rotation velocity or a rotation frequency of a rotatory material. The touch-type of device has some problems on measurement accuracy, response time, difficulty in use, durability and manufacturing. On the other hand, for example, a laser-type sensor and an electroacoustic-type sensor belong to the untouch-type of device. The laser-type sensor including a semiconductor position-sensing device is mainly used for measuring a vibration displacement along the direction vertical to the laser beam applied to a material. The laser-type sensor has a defect that the longer the length of the laser beam, the lower the measurement accuracy because of flickering of the laser beam itself. In addition, the use of the laser-type sensor is impossible for the measurement in opaque media. The electroacoustic-type sensor is used for measuring the vibration displacement in a material near a terminal of a pipe. The electroacoustic-type sensor is easy to be affected by a change in circumstances, and has some problems on measurement accuracy, and so on.
An object of the present invention is to provide a sound pressure sensing device capable of sensing a sound pressure caused by touching a diaphragm constructed on the device with a high sensitivity.
Another object of the present invention is to provide a sound pressure sensing device capable of operating at a high frequency.
Another object of the present invention is to provide a sound pressure sensing device capable of transducing a sound pressure to an electric signal.
Another object of the present invention is to provide a sound pressure sensing device excellent in measurement accuracy, response time, durability, manufacturing.
Another object of the present invention is to provide a sound pressure sensing device which is not affected by a change in circumstances, for example, a change in temperature.
A still other object of the present invention is to provide a sound pressure sensing device easy in use and having a small size which is very light in weight and has a simple structure.
According to one aspect of the present invention there is provided a sound pressure sensing device comprising a piezoelectric substrate having two end surfaces, an input interdigital transducer, a first output interdigital transducer, a second output interdigital transducer, a diaphragm having an inner- and an outer surfaces, a liquid tank, and a signal analyzer. All the input-, the first output-, and the second output interdigital transducers are formed on one end surface of the piezoelectric substrate. The liquid tank has a liquid in contact with the other end surface of the piezoelectric substrate and the inner surface of the diaphragm.
If an input electric signal is applied to the input interdigital transducer, an elastic wave is excited in the piezoelectric substrate. A leaky component of the elastic wave is radiated effectively in the form of a longitudinal wave into the liquid. The longitudinal wave is reflected in the liquid by the diaphragm. A reflected longitudinal wave is detected at the first output interdigital transducer as a first delayed electric signal. A non-leaky component of the elastic wave is detected at the second output interdigital transducer as a second delayed electric signal. A sound pressure caused by touching the outer surface of the diaphragm is sensed by the signal analyzer from a difference between the first- and second delayed electric signals.
According to another aspect of the present invention there is provided an amplifier connected between the input interdigital transducer and the second output interdigital transducer. A part of the second delayed electric signal is amplified via the amplifier, and is fed back as the input electric signal again. Thus, the input interdigital transducer, the second output interdigital transducer and the amplifier form a self-oscillation type of delay-line oscillator.
According to another aspect of the present invention there is provided an amplifier connected between the input interdigital transducer and the first output interdigital transducer. A part of the first delayed electric signal is amplified via the amplifier, and is fed back as the input electric signal again. Thus, the input interdigital transducer, the first output interdigital transducer and the amplifier form a self-oscillation type of delay-line oscillator.
According to another aspect of the present invention there is provided a signal analyzer comprising a phase comparator, which compares an acoustic phase delay of the first delayed electric signal with that of the second delayed electric signal. Thus, a sound pressure is sensed in terms of a phase difference between the first- and second delayed electric signals.
According to another aspect of the present invention there are provided an input-, a first output-, and a second output interdigital transducers having an arch-shape, respectively, and arranged to have one concentric center.
According to another aspect of the present invention there is provided a piezoelectric substrate made of a piezoelectric ceramic thin plate, of which the polarization axis is parallel to the thickness direction thereof
According to another aspect of the present invention there is provided a piezoelectric polymer thin plate.
According to another aspect of the present invention there is provided a diaphragm made of a polymer film.
According to other aspect of the present invention there is provided a diaphragm made of a metal film.
According to a further aspect of the present invention there is provided a sound pressure sensing device comprising a piezoelectric substrate having two end surfaces, an input interdigital transducer, a first output interdigital transducer, a second output interdigital transducer, a tank, and a signal analyzer. All the input-, the first output-, and the second output interdigital transducers are formed on one end surface of the piezoelectric substrate. The tank has a liquid room with a liquid in contact with the other end surface of the piezoelectric substrate, an air room with a diaphragm panel having an inner- and an outer surfaces, and a partition wall between the liquid- and air rooms.
If an input electric signal is applied to the input interdigital transducer, an elastic wave is excited in the piezoelectric substrate. A leaky component of the elastic wave is radiated effectively in the form of a longitudinal wave into the liquid. The longitudinal wave is reflected in the liquid by the partition wall. A reflected longitudinal wave is detected at the first output interdigital transducer as a first delayed electric signal. A non-leaky component of the elastic wave is detected at the second output interdigital transducer as a second delayed electric signal. A sound pressure caused by touching the outer surface of the diaphragm panel is sensed by the signal analyzer from a difference between the first- and second delayed electric signals.